If $A = \left[\begin{array}{ccc} 1 &-6 &1\cr 0 &1 &-6\cr 0 &0 &1 \end{array}\right]$, then

$A^{-1} =$ $\left[\Rule{0pt}{3.6em}{0pt}\right.$$\left]\Rule{0pt}{3.6em}{0pt}\right.$.